Gaseous discharge tube cathode



Dec. 29, 1936. D. v. EDWARDS ET AL 2,065,997

GASEOUS DISCHARGE TUBE CATHODE Filed June 22, 1934 Fig.

INVENTORS ATTORN EYS Y Patented Dec. 29, 1936 UNITED STATES GASEOUSDISCHARGE TUBE CATHODE Donald V. Edwards, Montclair, and Earl K. Smith,East Orange, N. J., assignors to Electrons, Inc., of Delaware, acorporation of Delaware Application June 22, .1934, Serial No. 731,860

11 Claims.

This invention relates to gaseous discharge devices, and is particularlyapplicable to such devices having thermionic cathodes of theheatshielded type. It is also useful in connection 5 with tubesemploying gaseous pressures of a low order.

The art has experienced difficulty in the use of gaseous dischargedevices, especially those employing pressures below one millimeter ofmercury and down to about five-thousandths of a millimeter (.005 mm.)and those in which the shields and glass walls are subjected to ionbombardment. The starting voltage and are drop in such devices have beenfound to increase with use and if the voltage is increased to forcestarting the difficulty is aggravated until finally only a glowdischarge occurs. This amounts to a complete failure to functioninasmuch as a glow, discharge has high resistance and cannot carry loadcurrents. Such failures occur even though the electrodes are apparentlyin good condition and the generally accepted explanation has been thatthe gas has disappeared; hence the difliculty has been called hardening.

We have found that this so-called hardening is not caused by gasdisappearance but, on the contrary, is due to cathode conditions broughtabout by a lack of sufficient free space for ionization immediatelyadjacent the cathode emissive surface, or surfaces, whereby the samecannot be utilized effectively. When there is actual disappearance ofgas it is a secondary effect.

The object of the invention is to avoid the difliculty above-mentioned.To this end We arrange the cathode according to the pressure and kind ofionizable medium in a gaseous discharge device so that there will besufiicient free and unobstructed space adjacent the emissive surface toutilize its emission efiectively for the generation of ions.

Further and related objects of the invention are the provision of acathode which will operate satisfactorily in low-pressure tubes, and theprovision of a high-voltage gaseous rectifier having longer life thanhas heretofore been practical.

The invention will be described with reference to the accompanyingdrawing in which Fig. 1 shows, partly in section, a gaseous dischargetube employing the invention; and Fig. 2 shows a modified form ofcathode.

In Fig. l of the drawing, l is an anode, and a control grid 2 may or maynot be provided for timing the starting of the discharge. 3 is thecathode which, in the form shown, consists of a cylindr cal hollow bodyopen at the top and having its inner surfaces treated to render themelectron emissive. The cathode is indirectly heated to operatingtemperature by a. heater 4. Heating energy is conserved by a heat shieldsurrounding both the heater and cathode, and consisting of a number ofnested, heat-reflecting cans 5 having perforated covers 1. The cathode3, heater 4, covers 1, and cans 5 are connected together near the top ofthe cathode. The above described electrodes are contained in an enlelope8 which is provided with the usual re-entrant stems and exterior caps orbases. The anode l is supported from the upper stem by lead-in wiressealed therein and connected to a terminal 9 in the upper base.Connection is made to the grid! through an additional lead-in wire whichis connected at one end to a terminal I!) and at its other end to across bar ll (shown in section) this cross bar being secured to two offour gridsupport wires I4 which are fastened at their upper ends to acollar (not shown) around the upper stem. A similar collar (not shown)around the lower stem, and wires l5 support the outer can 5 and theother parts secured thereto. Lead-in wires l6, secured to the outer can,provide additional support and also provide a common,cathode-heater-shield connection to a ter minal I2 in the lower base.The lower end of heater 4 is welded to a disc 6 in such manner as toallow for expansion of the heater, the disc being connected by insulatedlead-in wires II to terminal l3.

The distance (1 represents the free space for ionization adjacent theemissive surface of cathode 3; it is the distance an emitted electronmay travel before it strikes the surface of a solid body, such as theopposite emissive surface. In the form of cathode shown in the drawing,d is the inside diameter of the cathode 3 and this distance is availableto electrons emitted from any part of the cylindrical surface. An equalor greater distance is available to electrons emitted from the bottomsurface of the cylinder. Sometimes a cathode is provided with vanes toincrease the emissive area in a given space, or the cathode may havesome other form or arrangement of its surfaces.

In such cases the distance contemplated by the present invention is thatof the unobstructed space between oppositely disposed emissive surfaces,or it is the distance from an emissive surface, and perpendicularthereto, to another surface whether emissive or non-emissive, the latterincluding neutralizing and shielding surfaces which may or may not beconnected to the cathode.

The art has endeavored to provide a maximum of emissive surface in aminimum of space in order to take advantage of the ability of electronsto travel in curved paths in an ionized medium. However, we have foundthat sufficient space should be provided for generating the ions andthat there is a minimum value for the distance d in a given tube if theaforementioned difficulties are to be avoided. This minimum depends uponthe kind of ionizable medium and its. pressure under operatingconditions, and to some extent upon the cathode emissivity. It ispreferable for practical reasons, such as conserving the heating energyand making the tube as small as possible consistent with other factors,to design the cathode for the minimum value of d plus a factor ofsafety, inasmuch as no advantage is gained by making d too large.

According to the invention we make the distance d not less than the sumof the distance an electron must travel to attain ionizing velocity plusthe average distance an electron must travel after that to produce anion. For brevity, we shall call the former the accelerating distance andthe latter the mean-free-path for ionization. Their sum is a distancewhich allows the average electron to travel freely from the cathodesurface to the first ionizing collision.

The accelerating distance varies with load and cathode activity and maybe estimated mathematically. Its magnitude need not be known accuratelyas it is a small part of the above sum for low pressures and substantialload currents. The accelerating distance may also be defined as theperpendicular distance from an emissive surface to the outer boundary ofits electronic space charge. Inasmuch asthe space charge is almostcompletely neutralized at full-load current, the boundary with suchcurrent is very close to the cathode surface. The space charge increasesand its boundary recedes from the cathode as the current is decreased,from which it will be apparent that the accelerating distance approachesinfinity as the current approaches zero. However, the currents which aresmall enough to make the accelerating distance large can usually besupported safely by a small portion of the cathode surface, such as theedge portion nearest the anode. In determining the accelerating distancefor the type of tube shown in the drawing, it is sufficient to take thecurrent at about 20% of full-load current. In some tubes a dark sheathis visible adjacent the cathode surface at low loads. If the emissivesurface is visible through the tube envelope and if such a sheathappears, then all loads less than that at which the sheath becomesperceptible may be disregarded in determining the accelerating distancefor that tube.

As an example of the length of accelerating distance which may beexpected with a cathode of ordinary activity, let us assume a tubefilled with argon at 0.1 millimeter pressure. The instantaneous peak orcrest current in such a tube at full load would give an acceleratingdistance of about 0.01 centimeter, whereas at 10% load the acceleratingdistance would increase to 0.1 centimeter. It will be understood ofcourse that, during each cycle of full-load current, the acceleratingdistance will vary from the above minimum of 0.01 centimeter to amaximum value corresponding to minimum current for the cycle. However,in the example given the accelerating above life tests.

distance will not exceed 0.1 centimeter for any load that would causedisintegration of the oathode.

The electron mean-free-path for ionization is the more important part ofthe distance d. After an electron has been accelerated to or above thevelocity necessary for ionization in the p'articular medium, it usuallytravels a relatively large distance before it ionizes an atom,notwithstanding that it may collide with a number of atoms within thisdistance. Hence the mean-free-path for ionization should not beconfusedqwith the mean-free-path of an electron at the same velocity.The latter is the distance such an electron travels before it collideswith an atom, but, on the average, only one out of many such collisionsgenerates an ion. In a sense the ratio of the mean-freespath of anelectron to the meanfree-path for ionization is a measure of theefficiency of the emitted electrons in ionizing the gas. It is this lowefficiency which requires sufficient unobstructed space immediately infront of the emissive surface to give the electrons ample opportunity toionize atoms before they waste their energy by striking a solid body.

Some data as to the mean-free-paths for ionization have been given inpublications but values for gas pressures greater than .05 mm.are'scarce. Values obtained by exterpolating such data vary considerablyfrom the values which we have determined experimentally. Thisdiscrepancy is due to the increasing probability of cumulativeionization as the pressure increases. Our experiments with argon at 0.1mm. pressure would indicate a mean-free-path for ionization of about 1.6cm. for ionizing electrons of the velocity usually encountered in hotcathode discharges,

* whereas exterpolation of the published data would put the path atapproximately 3.0 cm. for

argon at the same pressure. However, both val-.

ues are several times greater than the meanfree-path of an electronwhich, for argon under the same conditions, is about 0.16 cm.

The mean-free-paths above-mentioned should not be confused with thekinetic mean-free-path of the argon atom, which pathis also smallrelative to the mean-free-path for ionization at the same pressure. Thekinetic mean-free-path is the average distance an atom moves due tothermal agitation before it collides with another atom.

The foregoing explanation of the distance d treats its two partsseparately and is based on present electron theory. The sum or totalvalue for d may be determined empirically by either of the followingmethods.

The first method consists in constructing a series of similar tubeshaving the same perpendicular distance d but with differentmean-freepaths obtained by filling the respective tubes with the samegas at different pressures. These tubes are put on life test and thepressure found below which some of the tubes develop the effectheretofore referred to as hardening. The said distance d is then theminimum distance for that gas and pressure and it may be used for futuredesigns, employing the same gas, pressure and cathode emissivity.

The second method is preferably employed after some experience has beengained with the It consists in operating a given tube on the pump atvarious gas pressures and plotting a curve of gas pressure versus arcdrop. The are drop should be fairly constant with decreasing pressuredown to a point at which it begins to increase rapidly. At this pointthe pressure is such that the distance d in the tube under test equalsthe mean-free-path for ionization plus the accelerating distance.

It is sometimes difficult to obtain definite and accurate results by theabove methods unless the proper load current is employed, which ingeneral should be relatively light, and some experience may be requiredin selecting the proper current. It should be large enough to shortenthe life of the tube by hardening but not so large that an abundance ofions is generated by the small percentage of electrons which succeed inhaving ionizing collisions with atoms in less than the average distance.For this reason a tube which is operated at full-load current, or one inwhich the current wave has normal average value but a high peak value,may develop hardening less readily than if it were operated most of thetime at about 30 to 50% of full-load current. The relative efi'ects ofload currents vary in different tube constructions, and depend 'toagreat extent upon the amount of shielding and whether the distance d isonly slightly below the minimum or very much smaller than it should be.In the latter case hardening may develop at any load greater than thatwhich can be supported by the small percentage of cathode surface whichis near the top and therefore substantially open to the tube atmosphere.

We believe that hardening is due fundamentally to a shortage of ions forthe needs of the arc discharge and that the discharge current isuniformly distributed over the cathode surface so long as the free spacemeets the above-described minimum. If the distance d is less than thisminimum for the particular gas and pressure used, there will be ashortage of ions under some load conditions. This causes the current toconcentrate on the top part of the cathode and disintegrate it. Thecurrent then concentrates, if possible, on another part of the cathodewith the same result, until the tube fails by, loss of emission insteadof by loss of gas as has generally been supposed. With this type offailure the cathode is not physically destroyed as in the case offailure due to long life with load or due to overloads. On the contrary,a cathode which has failed due to hardening has a low emission which, ifmeasured at a plate voltage below the ionization potential of the gasfilling, will probably have the same value as that of a new tube, butthe saturation emission of such a cathode is so low that it cannot carryload currents.

Apeculiar effect of making the distance 11 less than the minimum asabove described, in a tube having a heat-shielded cathode, is apronounced tendency to generate parasitic oscillations in arcdropvoltage at a frequency of approximately 10 kilocycles per second. Thisis probably due to the inability of the cathode to generate suflicientions inside the shield whereupon ions are generated outside thereof and,due to the high mobility of ions in this region, they suddenly diffuseinto the space within the shield thereby momentarily lowering the arcdrop. When the electrical charges on these ions are dissipated there isanother shortage and the process repeats itself at the frequencyabove-mentioned. The provision of a proper free space for ionizationwithin the cathode shield avoids this difiiculty.

In the event directly-heated emissive filaments are used, such as wherea number of spiral filaments are arranged with their axes parallel toone another about a circle inside a cylindrical heat shield, theeffective cathode surface for a large range of load will be the same asif the emissive surface were a continuous cylindrical surface passingthrough the axr s of the spirals.

Such a cathode is illustrated in Fig. 2 which shows a horizontal sectionof a cylindrical heatshield 20 containing a number of spiral emissivefilaments 2| disposed in a circle of diameter d with their axes parallelto each other and to the cylindrical surface of the shield. Thefilaments may have their upper ends connected to the shield and theirlower ends connected to an insulated disc, such as 6 in Fig. 1.According to the invention the distance d which, as stated, is thediameter of the eiTective cathode surface, should be determined asdescribed above. Under normal loads no ionization goes on between adjacent spirals, and there may be other arrangements whereby part of theemissive surface is ineffective, hence the effective cathode surfacewill be smaller in such cases than the total emissive surface. However,the effective surface should be suificient for the operatingrequirements of the tube and should have suflicient free space forionization adjacent thereto.

Where a directly heated filament has a material voltage drop along itslength, it is possible to reduce the free distance somewhat provided thefilament voltage is phased, relative to the anode voltage, so that themore distant end of the filament is negative, relative to the end nearerthe anode, on the half-cycles when the anode is positive.

The invention is particularly advantageous in making high-voltage tubes,for it is known that the permissible anode voltage for most rectifiersincreases as the pressure of the gas filling is reduced. The permissibleanode voltage remains fairly constant with decreasing pressure down to acritical pressure below which it increases rapidly. This criticalpressure can be raised by decreasing the physical size of the tube.However, former low-pressure tubes intended to take advantage of theserelations have had very short life. We have found that this is due to alack of a sufficient space for ionization which, according to ourinvention, may be made large enough so that the pressure at which thearc drop suddenly increases is below the critical pressure at which thepermissible anode voltage suddenly increases.

The following is a specific example of the results thereby obtainable. Atube having only 1.2 cm. of free space may be filled with not less than0.4 mm. of argon or roughly 0.2 mm. of xenon, and have a permissibleplate voltage of volts per anode. By merely increasing the free distancefor ionization to 2.8 cm. the same tube may be filled with 0.05 mm. ofargon or 0.025 mm. of xenon and have the same current rating but seventimes greater plate voltage rating. Thus with slightly more thantwo-fold increase in the cathode free space and without increasing thesize of the tube, a seven-fold increase in permissible tube output isobtained.

From the foregoing it will be apparent that the invention isparticularly applicable to tubes containing metal vapor, such as mercuryvapor tubes. In such tubes the vapor pressure is not constant but variesover a wide range due to variations in load and ambient temperature. Thecathode in such a tube should be designed to have a free distance forionization, as above described, for the lowest ambient temperature andthe smallest load for which the tube is designed. If the distance islarge enough under these conditions it will be ample for highertemperatures and greater loads.

The-invention may be employed with other rare gases and vapors thanthose specifically mentioned and also with common gases if they areinert to the envelope and the electrodes.

We claim:

I 1. A discharge device comprising an envelope containing an ionizablemedium, a hollow thermionic cathode having a fixed electron emissivesurface therein, and another electrode outside the cathode, said cathodehaving an unobstructed space adjacent its effective emissive surface fora distance which allows the average emitted electron to move from saidsurface to its first ionizing collision in said medium without strikinga solid body.

2. A discharge device including a gas filling, an anode, shields, and athermionic cathode having a fixed electron emissive surface, saidcathode having an unobstructed space in front of its effective emissivesurface for a distance which bears the same ratio to 1.6 centimeters asthe mean-free-path for ionization in said filling bears to themean-free-path for ionization in argon at 0.1 millimeter of mercurypressure.-

3. A gaseous discharge tube having an anode, a thermionic cathode havinga fixed electron emissive surface, and a gas or vapor filling at a lowerpressure for the particular filling than the pressure which gives amean-free-path for ionization in said filling corresponding to themeanfree-path in argon at 0.2 millimeter of mercury pressure, and aheat-shield cooperating with said cathode, the cathode having freeperpendicular distances from substantially all of its effective emissivesurface to other surfaces of said cathode and shield, said distancesbeing greater than the average distance an electron must travel from thecathode to its point of "ionization for all loads down to that at whichthe cathode has a perceptible dark sheath.

4. In a discharge device containing an ionizable medium at low pressure,a hollow cathode structure having a fixed electron emissive surface andunobstructed space therewithin, measured .perpendicularly tosubstantially all of the effective emissive surface, sufficient to allowthe average emitted electron free travel to its first ionizing collisionin said medium at any load great enough to affect the cathode life, saidcathode structure having a discharge opening which maintains theionizable medium inside the hollow cathode at the said low pressure.

5. The combination in a gaseous discharge tube of an anode, a thermioniccathode having a fixed .electron emissive surface, a filling of gas orvapor at low pressure, and shields, said cathode having a space in frontof substantially all of its effeccathode, and another electrode, saidcathode having oppositely disposed fixed,'electron emissive surfacesspaced apart, over substantially their entire area, a distance at leastas great as the sum of the electron accelerating distance forsubstantial load currents in said medium plus the electronmean-free-path for ionization therein, the space between said emissivesurfaces having free communication with the remaining space in saidenvelope.

'7. A gaseous discharge tube comprising'an envelope, an ionizable mediumat a pressure of about 1 millimeter of mercury or less and a pair ofcooperating electrodes in said envelope, one electrode being athermionic cathode having a fixed electron emissive surface and anunobstructed space, immediately adjacent its emissive surface, at leastas great as the sum of the electron accelerating distance plus themean-free-path for ionization in said medium, said unobstructed spacehaving free communication with the remaining space in said envelope.

8. A gaseous discharge tube comprising an envelope, an ionizable mediumand a pair of cooperating electrodes in said envelope, one electrodebeing a hollow thermionic cathode containing fixed electron emissivesurfaces, the distances between said surfaces being fixed according tothe kind and pressure of said medium so as to provide sufilcient freespace for the majority of the emitted electrons to ionize said mediumwithin the cathode, said cathode being open to said medium whereby thepressure within the cathode is not increased relative to the pressureoutside thereof.

9. A discharge device comprising an envelope, an ionizable mediumtherein, and electrodes including a thermionic cathode having a fixedelectron emissive surface, a heat-shield therefor and another electrode,said electrodes and shield being so disposed within said enveloperelative to the effective emissive surface of the cathode that there issuflicient distance to permit the average electron to travelperpendicularly from said surface to its first ionizing collision withan atom of said medium before striking one of the said electrodes, thespace between the cathode and said other electrode being unobstructed.

10. A discharge tube comprising an envelope containing argon atapproximately .05 millimeter of mercury pressure, an anode, a thermioniccathode having fixed electron emissive surfaces and a free distance forionization of about 2.8 centimeters between said emissive surfaces, anda heatshield surrounding said cathode, said tube having a permissibleanode voltage of at least 750 volts.

11. A discharge tube comprising an envelope containing a rare gas, ananode, a thermionic cathode having fixed electron emissivesurfaces, anda heat-shield surrounding said cathode, the said emissive surfaces beingspaced apart a distance greater than the electron mean-free-path forionization in said gas, the pressure of said gas being above thepressure at which the arc drop in said tube increases rapidly and belowthe critical pressure at which the permissible anode voltage suddenlyincreases.

DONALD V. EDWARDS. EARL K. SMITH.

